Functional
imaging of the brain demonstrates that this highly complex organ adapts
to injury by redistributing its cognitive workload across established
neural networks and recruiting local cortical areas to fill in for lost
functions like speech and language comprehension.

"Functional
MRI [magnetic resonance imaging] indicates that the dogma that some
areas of the brain … are not important for normal function is clearly
fallacious," said Dr. Keith Thulborn, director of MR research at the
University of Illinois at Chicago at the annual meeting of the American
Association for the Advancement of Science in San Francisco. "Loss of
any brain tissue is likely to compromise the reserve capacity of many
large-scale neurocognitive networks that will ultimately be reflected
in the performance of more difficult tasks or recovery from subsequent
disease processes."

Watching
the brain at work with a very-high-field MRI scanner, Thulborn has mapped
a two-stage recovery process in patients who lost their language skills
after strokes. In one patient suffering from damage to Wernicke's area
(the region in the left cortex that controls the understanding of language),
functional MRI showed that the brain initially recouped by allocating
speech comprehension to an area on the opposite side of the brain. Over
time, while Wernicke's area remained damaged, an adjacent area took
on this cognitive task.

The
ability of the brain to maintain performance by recruiting undamaged
portions of the cortex may suggest why functional recovery can occur
even after large strokes, said Thulborn.

One
key factor in recovery time, Thulborn suggested, is whether white matter
has been damaged. White matter consists of myelinated neuronal axons
that serve as cables linking the different areas of the cortex. When
these are injured, vital connections needed to allocate functions elsewhere
are lost.

"The
involvement of white matter tracts portends slower and reduced recovery,"
said Thulborn. "This may reflect reduced capacity to redistribute workload
when the connectivity through white matter is disrupted."

While
functional MRI has been largely used in research to map brain functions,
it is just beginning to find clinical applications. At the UIC Medical
Center, Thulborn is collaborating with other physicians, psychologists
and therapists to use the technology in designing and monitoring rehabilitation
programs aimed at restoring lost cognitive and motor skills. In this
role, Thulborn said, functional MRI can guide and refine therapies to
enhance the brain's innate plasticity.

The
very-high-field MRI scanner works by picking up faint magnetic signals
in the underlying tissue. As neurons become increasingly active in specific
regions of the brain, blood flow surges to those regions and blood volume
expands.

In
the process, deoxygenated blood is replaced with oxygenated blood, the
two differing in their magnetic properties. The MRI scanner is able
to detect this magnetic change, although minute, because of the scanner's
high magnetic field strength: 3.0 Tesla, twice that of MRI scanners
more commonly deployed in clinical settings. (A Tesla is equivalent
to 10,000 gauss; the magnetic field strength of Earth is less than one
gauss.) Cross-sectional images are made through the entire brain to
create a three-dimensional view. The images must be run through a series
of statistical programs so that they can be correctly interpreted.

To
obtain images of the working brain, patients are placed on a table and
moved into the center of the magnet. The images are taken while patients
are engaged in a set of cognitive tasks devised to correlate functional
activities with specific areas of the brain. To map the language comprehension
network of the brain, for example, patients are given sentences of varying
complexity to read and asked to answer true/false questions by pressing
a finger switch. To map the motor and sensory areas, patients simply
tap a finger.

Thulborn
cautioned that these cognitive tasks must be carefully selected if they
are to be of value in answering clinical questions. Moreover, they need
to be "robust," or reproducible, and appropriate to the patient's level
of education and cognitive and motor abilities. "Careful attention to
matching the skills of each patient to the stimulus task is required
to avoid variable performance that may alter the functional mapping,"
said Thulborn.